The mold determines the contours of a die cast part and affects its properties.
Excerpt from the Spotlight Metal March 2019 article by Isabell Page.
Die casting is a forming process for the mass production of, parts made from aluminum, magnesium, 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 mold where it takes the shape of the die and solidifies. These die casting molds consist of two halves so the cast part can be removed from the mold. The feed-side mold half is mounted on a fixed plate on the rigid side of the die casting machine, while the ejector-side mold 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 mold 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 mold is closed, the melt is pressed into the mold under a pressure of up to 1,200 bar, achieving maximum mold filling speeds of 150 m/s (540 km/h) [1]. High closing and clamping forces are required to press the mold halves against each other and keep the molds 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 molds 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 mold halves open and the cast part is ejected by bolts or removed by a robot and conveyed for further processing.
High Performance Steels
A central issue regarding the die casting process is the mold. It determines the contours that have to be transferred to the cast part and should also enable the cast part to solidify as quickly as possible. In this way, the formation of a fine-grained microstructure is promoted, which is beneficial to the casting quality. In order to achieve optimal cooling, the molds are cooled in certain parts. Another effect is that the production time is shortened, which provides economic advantages. The design of die casting tools is described in the standard DIN 16760-1 [2]. The tools used in the die casting process are inevitably exposed to high thermal and mechanical loads and must be able to withstand them permanently. For example, molds for zinc die casting reach service lives of 500,000 to 2 million cycles. In order to achieve such performances, the die casting tools, which besides the above mentioned molds, include mold inserts, cores, slides and ejectors, are made of high-strength hot-work steels such as X40CrMoV5-1 (1.2344) or special materials, for example hard metals. Properties that play a very important role in these tools are high wear resistance, high ductility, high heat resistance, high hot tearing and hot wear resistance and good thermal conductivity. When choosing the materials, their technological properties, the design of the tools, their heat treatment and last but not least the complex interactions between the tools and the metal to be cast must be considered. For this purpose, the manufacturers and suppliers of the appropriate steels offer informative brochures and consulting services [3].
