Pressure die casting is a metal forming process characterized by the pressurization of the cast materials within a die. The die casting process often ranges in pressure depending on the type of metal, its melting point and the thickness and complexity of the mold.
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Die casting is exactly the same as permanent casting except for its higher operational pressure levels, which can range anywhere from 1,450 to 30,500 PSI; for this reason, die casting is a fairly energy-intensive process. The first ever die castings involved low operational pressure, but advances in technology made higher pressures more easily achievable. The two different types of die casting, cold chamber and hot chamber, differ in terms of construction and process as well as in terms of working pressure level. Cold chamber die casting machines have a holding pot that is separate from the furnace. They are used for metals with high melting points that require intense heat, which could cause damage to the plunger, cylinder and holding pot. Instead of continuous flow, the molten metal is ladled from the furnace into the pot, never coming into direct contact with a plunger. This type of die casting requires pressure levels between 2,000 and 20,000 PSI, while hot chamber die casting requires anywhere from 1,000 to 10,000 PSI. Hot chamber die casting, on the other hand, is used when the metal's melting points aren't very high. The holding pot is located directly inside the furnace, and continuous flow is allowed.
High and low pressure die casters are constructed slightly differently. During high pressure die casting, the liquid metal is injected at high speeds. Two vertical platens contain bolsters that hold the die halves - one that is fixed in place, and one that is able to open and close. The metal is poured in from the holding pot and shot into the mold by a hydraulic piston. After the metal solidifies, the mold opens and the metal part is ejected. During high pressure die casting, blistering caused by gas inclusions is a common concern. Since this process takes more energy, it is only cost effective for mass produced items. In a low pressure die caster, the holding tank containing the metal is located below the mold. The pressure inside the die is decreased by a vacuum pump. This difference in pressure between the mold and the holding tank causes the metal to flow upwards into the die. While high pressure die casting produces more complex parts, low pressure die casting is specially designed for symmetric, rounded parts like automotive wheels.
