What is aluminum pressure die casting technology?

Aluminum pressure die casting technology is used in the production of complex-shaped products from aluminum materials. This method requires almost absolute precision to ensure the quality of the product. 

There are two common aluminum pressure casting methods: high pressure casting and low pressure casting. 

• High pressure die casting: High pressure die casting is a method that uses piston pressure from pistons to compress metal in a mold.
• Low pressure die casting: Low pressure die casting is a method of vacuuming the inside of the mold to accelerate the solidification and hardening of aluminum materials.

There are actually many common die casting methods. 

 Working principle of aluminum pressure die casting technology

Pressure high pressure die casting technology is the process where aluminum metal is filled into a mold and solidified under the action of pressure. 

The mold is composed of 2 halves. Initially the two halves of the mold are closed. The worker will fill the molten into the press chamber. During chamber die casting process, the piston will be in charge of forcing the liquid aluminum down. Liquid aluminum under the action of pressure will slowly flow down the parts of the part to fill the mold.

One point to note is that this process must take place very quickly and under high pressure.

This pressure level continues to be maintained until the aluminum metal solidifies. In the final step, the craftsman will remove the intestines from the mold and take the casting out. This is the complete process of high pressure die casting method.

In pressure high pressure die casting technology, the die is designed according to a static and dynamic structure. When casting is complete, the dynamic mechanism will assist in removing the casting from the mold. The static mechanism is in charge of assisting in removing the excess aluminum from the piston cylinder mouth.

The essence of the low pressure die casting method

They consist of the fact that the filling of the gravity casting with a melt and the solidification of the casting material occurs under the influence of excess pressure of air or gas. Shaping can be carried out in a mold, sand, or a combination mold (chill and sand or shell cores), as well as in a ceramic or shell mold.

 The technological scheme of casting under low pressure die casting from which it can be seen that if compressed air or gas is supplied inside the sealed chamber a, under pressure Pex > Patm, then due to the resulting pressure drop, the liquid metal will rise along the metal wire 1 and fill form 2 to the level of the corresponding H = (Pex. – Pathm.)/ ρ. 

The term “low pressure die casting ” is used because an overpressure of less than 0.1 MPa is required to lift the melt and fill the mold.

Advantages of the low pressure casting method

• The ability to control the melt flow rate in the mold cavity by controlling the pressure in the installation chamber is especially important for improving the filling of molds of thin walled castings.
• The presence of excess pressure on the melt during crystallization provides increased power to the casting and entails an increase in the density of the casting.
• Possibility of automating the time-consuming operation of pouring a mold.
• Reduction of metal consumption for the gating-feeding system due to draining back into the crucible of unhardened metal from the metal pipeline, which increases the yield factor (in some cases – up to 90%).

Low pressure casting is most widely used for the production of complex-shaped and especially cold chamber die casting from aluminum and magnesium alloys.

Die casting is done by machine into metal molds called molds

Filling the mold with metal is carried out after it is closed through the sprue channels, which connect the working die cavity of the mold with the pressing chamber of the die casting machine.

The outer outlines of the casting are formed by the working surface of a closed mold, while the internal holes and cavities are obtained using rods that are removed from the hardened casting at the moment the mold opens.

The filling of the mold cavity with the melt and the solidification of the casting occurs under the action of excess pressure of air or gas. Similarly, it also impacts on the dimensional accuracy of the wall thickness of the molten metal alloy.

There are 2 types of die casting machines – compressor and piston.

Piston machines with cold and hot chambers are the most widely used.

The hot pressing chamber is located inside the crucible with molten metal, and the cold chamber is separate from the molten metal and is located horizontally or vertically.

Modern die casting machines have a fully automated CNC workflow; the adjustment of the clamping force of the molds, the change in the pressing speed, and the blocking of the nodes in case of malfunctions in the machine are automated.

Controlled pressure casting creates a wide range of possibilities for controlling the process of filling the mold with melt.

If compressed air or gas is supplied inside the sealed chamber under pressure Psb>Patm, then due to the pressure difference, the melt will rise along the metal pipeline and fill the mold to a level corresponding to  H=(Psb-Ratm)  /  P .

This filling method is called low pressure metal casting

The term “low pressure” is used because the required overpressure is less than 0.1 MPa to lift the melt and fill the mold.

The impact of pressure on the solidifying melt makes it possible to improve the feeding conditions, reduce the volume of shrinkage defects, and improve the quality of the casting – mechanical properties and tightness.

In the processes under consideration, after filling the mold, the pressure acts on the melt, which from the crucible through the metal conduit enters the solidifying casting and feeds it.

Due to this, the shrinkage porosity in such castings decreases, and the density, and mechanical properties increase.
Low-pressure casting is carried out in plants in such a way that the process of filling the mold with melt – the most time-consuming and unpleasant operation from the point of view of health and safety – is carried out automatically.

The designs of plants and machines for these foundry processes also provide automation of the assembly and opening of molds, ejection of the casting, and its removal from the mold.

Thus, the processes of molding under controlled pressure can improve the quality of high pressure die castings and provide automation of their production.

Die casting process

The die casting process includes the crucible with the melt in the distributing furnace (pressure chamber) of the installation is hermetically sealed with a lid in which a metal conduit made of a heat-resistant material is installed.

The metal pipeline is immersed in the melt so that its end does not reach the bottom of the crucible by 40-60 mm.

The mold mounted on the lid is connected to the metal pipeline with a sprue bushing.

The cavity in the casting can be made with a metal, shell, or sand core.

Air or inert gas under pressure up to 0.1 MPa through the control system enters through the pipeline into the installation chamber and the melt enters the mold from below through the metal pipeline, sprue, and manifold at a speed controlled by pressure in the installation chamber.

Upon completion of filling the mold and solidification of the casting, a valve automatically opens, connecting the installation chamber with the atmosphere.

The air pressure in the chamber is reduced to atmospheric and the unsolidified melt from the metal pipeline is drained into the crucible.

After that, the mold is opened, the casting is removed and the cycle is repeated.

The main advantages of the low pressure casting process are:

• automation of the labor-intensive operation of mold filling;
• the ability to control the melt flow rate in the mold cavity by changing the pressure in the installation chamber;
• improving the nutrition of the casting;
• reduction of metal consumption for the gating system.

Main disadvantages:

• low resistance of the part of the metal wire immersed in the melt, which makes it difficult to use this casting method for alloys with a high melting point;
• the complexity of the system for regulating the melt flow rate in the mold, caused by the dynamic processes occurring in the installation when its chamber is filled with air,
• instability of air leaks through the seals, lowering the level of the melt in the installation as castings are made;
• the possibility of deterioration of the quality of the alloy during long-term holding in the crucible of the installation;
• the complexity of operation and adjustment of installations.

Mold Requirements

The advantages and disadvantages of the method determine the rational scope of its application and prospects for use.

Low-pressure casting is most widely used for the manufacture of complex shaped and especially thin-walled castings from aluminum and magnesium alloys, simple castings from copper alloys, and steels in series and mass production.
The product can be removed from the mold at a higher temperature.

Products obtained by casting at low pressure are characterized by a low level of residual stresses and low warping.

The use of low pressures and low speeds drastically reduces the requirements for the mechanical strength of the mold parts, which makes it possible to significantly reduce the thickness of the plates and the weight of the mold compared to conventional casting. The mold can be made from inexpensive, easily machined materials. 

Casting defects and ways to eliminate them

    Gas shells in castings are formed due to a malfunction of the metal wire, insufficient volume of alloy in the crucible to fill the mold, high speed of filling the mold with metal, resulting in air entrapment, insufficient mold ventilation, and insufficient alloy refining.

To prevent the formation of gas shells in castings, it is necessary to monitor the serviceability of the metal pipeline and compliance with the casting process; the alloy must be carefully refined.

    Shrinkage cavities and porosity in castings are formed due to the unsatisfactory thermal regime of mold crystallization (the direction of crystallization to the sprue and to the profitable parts of the casting is not ensured).

About the features of casting formation

Filling the mold with melt in this casting process can be carried out at flow rates that can be controlled over a wide range.

To obtain high-quality castings, it is preferable to fill the mold with a continuous flow, at speeds that ensure high-quality filling of the mold and exclude the capture of air by the melt, the formation of gas shells in the castings, the ingress of oxide films and non-metallic inclusions into them.

However, a decrease in the flow rate necessary to maintain its continuity can cause premature cooling and solidification of the melt, i.e. until the form is completed.

Therefore, as in other casting processes, it is important to coordinate the hydraulic and thermal modes of filling the mold with the melt.

Depending on the combination of structural and pneumatic parameters of the installation, the movement of the melt in the metal wire and the casting mold during filling can occur both with an increasing flow rate and with its oscillatory change.