The process of high pressure zinc die casting can be divided into two methods. Cold chamber die casting and hot chamber die casting, and the major difference between them is the different type of machines that are employed. Usually, hot chamber machines are more suitable for small and light zinc products, while cold chamber die casting machines are usually used to manufacture large heavy die casting products.
The hot chamber die casting process has been around a bit longer than the cold chamber die casting process, and it is ideal for the casting of copper, lead and zinc alloys. The hot chamber type of machine is more suitable for for casting small size die products, and very commonly the hot chamber process will have a much faster speed than the cold chamber process. The hot chamber process is not really suitable for large and heavy die cast products.
During the hot chamber die casting process, the molten alloy is pressed into the die cavity, after the cavity has been filled the surplus molten alloy flows back to the furnace along with pressure release. With a small size die cast product, a hydraulic shot cylinder is applied which will finish the injection at once.
After the injection of molten alloy, the alloy filling the cavity cools with the application of a water cooling system, and the surplus alloy flows back to the furnace. Hot chamber die cast parts generally have a much smaller sprue, and faster casting speeds than the cold chamber die casting process. This type of die casting process is ideal for lead, copper and zinc alloys. For the production of zinc and zinc alloy castings the dies are generally manufactured from unalloyed steel, and for aluminium, copper, magnesium and the alloys of these metals, the dies are usually made from hot work tool steel because of its greater durability.
A complete cycle of the process will perform as thus: the die is closed and molten metal is forced into it, the cores are withdrawn, the die is opened, the casting is ejected and if necessary any shearing off the sprue and deburring the casting, then the die is cleaned. The number of cycles per hour that can be achieved depends on a number of different factors: casting metal used and size and shape of the casting.
With zinc alloys it is possible in a given period of time to produce many more castings per hour than it is to produce castings which are made from brass. Zinc pressure die casting makes possible the economical production of detailed castings at a very fast rate. The castings can comprise of details such as holes, recesses and so on, and are characterized by good surface finish, high dimensional accuracy and the economy of the metal. There are fully automated machines which can turn out thousands of small zinc alloy castings per hour. The machines may be pneumatically operated or the pressure may be developed by the action of a ram.