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Gravity Die Casting

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Gravity die casting is a process wherein the liquid metal is poured into metallic moulds without application of any external pressure. The liquid metal enters the cavity by gravity. Gravity die casting (GDC) is different from High Pressure Die Casting (HPDC), where the liquid metal is injected into the metal mould under very high pressures for production of thin walled smaller castings with better dimensional accuracy and surface finish.

Gravity die casting is a manufacturing process for producing accurately dimensioned, sharply defined, smooth or textured-surface metal parts. It is accomplished by gently pouring molten metal into reusable metal dies under the force of gravity. The term, "die casting," is also used to describe the finished part.

To begin the process, a cast iron mould capable of producing tens of thousands of castings must be made in at least two sections to permit removal of castings. These sections are mounted securely to a solid base and are arranged so that one is stationary (fixed die half) while the other is moveable (ejector die half).

To begin the casting cycle, the die caster clamps the two die halves tightly together. Molten metal is poured into the die cavity where it solidifies quickly. The die halves are drawn apart and the casting is ejected. Die casting dies can be simple or complex, having moveable slides, cores, or other sections depending on the complexity of the casting.

The main advantage of gravity die casting over sand casting is the high speed of production. The reusable die tooling allows for many hundreds of castings to be produced in a day. High definition parts reduce machining costs and superior surface finish reduces finishing costs.

Although die-castings are in most cases cheaper than sand castings, die tooling is considerably more expensive than sand tooling so an optimum number of castings need to be produced to make the process cost effective in the long run.

Advantages of gravity die casting

The process is suitable for mass production with better reproduction; dimensional accuracy and surface finish than conventional sand castings. A minimum wall thickness of 3.0 mm can be cast. Exceptionally, 2mm wall thickness is cast over small areas.
Castings ranging from few grams to 100 Kgs of Aluminium alloy can be cast. There are reports of some foundries producing cylinder blocks of around 300 Kgs by GDC. As the component size and complexity increases the process becomes more expensive and becomes uneconomical. It will also cause difficulty in handling the die and in extracting the casting from the die with reduction in dimensional accuracy and soundness of the casting.
The GDC process is capable of achieving 20% higher mechanical properties than that of a sand casting because of faster rate of solidification imparting better grain size. The process can be automated and also can produce semi-gravity die-castings employing sand or plaster of paris cores for production of interior details.

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