What is Die Casting

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

Usually, alloys with a low melting point are used. This casting process is particularly suitable for series and mass production of components because, unlike sand casting, for example, permanent metal molds are used which do not have to be destroyed after casting. It is possible to produce large and complex components with low wall thicknesses.

Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.

How Are Die Castings Made?

Hot-chamber die casting

Hot chamber machines are used for alloys with low melting temperatures, such as zinc, tin, and lead. The temperatures required to melt other alloys would damage the pump, which is in direct contact with the molten metal. The metal is contained in an open holding pot which is placed into a furnace, where it is melted to the necessary temperature. The molten metal then flows into a shot chamber through an inlet and a plunger, powered by hydraulic pressure, forces the molten metal through a gooseneck channel and into the die. After the molten metal has been injected into the die cavity, the plunger remains down, holding the pressure while the casting solidifies. After solidification, the hydraulic system retracts the plunger and the part can be ejected by the clamping unit.

Cold-chamber die casting

Cold chamber machines are used for alloys with high melting temperatures that can not be cast in hot chamber machines because they would damage the pumping system. Such alloys include aluminum, brass, and magnesium. The molten metal is still contained in an open holding pot which is placed into a furnace, where it is melted to the necessary temperature. However, this holding pot is kept separate from the die casting machine and the molten metal is ladled from the pot for each casting, rather than being pumped. The metal is poured from the ladle into the shot chamber through a pouring hole. The injection system in a cold chamber machine functions similarly to that of a hot chamber machine, however, it is usually oriented horizontally and does not include a gooseneck channel. A plunger, powered by hydraulic pressure, forces the molten metal through the shot chamber and into the injection sleeve in the die. After the molten metal has been injected into the die cavity, the plunger remains forward, holding the pressure while the casting solidifies. After solidification, the hydraulic system retracts the plunger and the part can be ejected by the clamping unit.

1. Clamping

The first step is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part.

2. Injection

The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine.

3. Cooling

The molten metal will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed.

4. Ejection

After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity.

5. Trimming

During cooling, the material in the channels of the die will solidify and attached to the casting. This excess material, along with any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing or using a trimming press.

Advantages of Die Casting

If you are reading this discussion on sand casting processes, you are likely trying to get a better idea of what method of prototype casting would be best for the product that you have in mind. Prototype casting using a sand casting foundry is best when you might need:

• Simple or complex shapes
• Lightweight
• High rates of production
• Corrosion resistance
• Monolithic – combine multiple functions in one
• Efficient and economical alternative to other processes
• Thin wall thickness – up to 0.040 inches for small castings
• Lightweight – light alloys are used
• Corrosion resistance – surfaces are smoother than other casting types