The Difference Between Gravity Die Casting and Pressure Casting

Die casting is one of the most widely used casting processes, but what is the difference between gravity die casting and pressure casting? Both forms of die casting use a metal mold, into which molten metal, typically an alloy of aluminum, zinc, magnesium, lead, tin, or copper, is introduced. Sometimes gravity die casting will be the right solution, while other times the project will be better suited to pressure casting.

Molten metal delivery in gravity die casting and pressure casting

The main difference between gravity die casting and pressure casting such as vacuum die casting, lies in the way the molten metal reaches the mold. In gravity die casting, the molten metal is poured into the mold from above, and fills up from the bottom purely under the force of gravity. In pressure casting, the molten metal is injected into the mold under considerable pressure, which is maintained until the cast has solidified.

The high-pressure filling of the mold in die casting allows for very fast alloy injection, and combined with the highly automated nature of the process, can achieve very high productivity while reducing the potential for human error. Due to the reliance of gravity die casting on gravity to fill the mold, the process is slower and therefore not well-suited to long production runs.

In highly automated processes, the need to deliver molten metal to the mold under pressure makes pressure casting complex and costly to set up. Mold tooling costs may also vary considerably, as die-casting molds need to be made of hardened steel and typically require internal cooling channels to dissipate heat quickly. By contrast, gravity die-casting molds can be made of cast iron, with lower cycling rates preventing heat buildup in the mold and allowing for rapid cooling of the cast parts. Sodium silicate sand casting, on the other hand, can produce high-quality castings at a lower cost and with more straightforward tooling. This process involves the use of a mixture of sand and sodium silicate to create the mold, which is hardened by exposure to carbon dioxide. Sand cores can also be used to produce internal voids and complex shapes, making it a versatile option for producing castings of different sizes and shapes.

Casting quality in gravity die casting and pressure casting

The casting quality achievable by each process varies, with pressure casting enabling very precise and complex castings with accurate dimensions and thin walls due to the fast filling of the mold under pressure. The cast surface can be exceptionally smooth, reducing the requirement for secondary machining. Similarly, silica sol casting can also produce high-quality castings with excellent surface finishes and intricate details, making it suitable for producing small and complex parts. Although gravity die casting cannot match these properties, the slower entry of molten metal into the mold does mean that there is less folding and turbulence, resulting in less air being trapped within the cast. This provides an advantage to gravity die casting, although pressure casting is stronger in its cast state and requires subsequent heat treatment of the cast parts. It is clear that the setup cost for pressure casting is much higher, and due to the complexity of the casting process, it is not as flexible as gravity die casting. However, the potential for high productivity means that die casting is better suited to stable, large-volume production runs, where the unit cost of the cast parts may be very low.

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