Die casting is one of the most widely used casting processes, but what is the difference between gravity die casting and pressure die casting? Both are casting processes used by Ming Dao, so we can do a good job of explaining the differences and discussing selection criteria.

  Both forms of die casting employ metal molds into which non-ferrous molten metal, usually an alloy of aluminum, zinc, magnesium, lead, tin or copper, is introduced. The main difference between gravity die casting (also known as permanent mold casting) and pressure die casting is the way the molten metal reaches the mold. In gravity die casting, molten metal is poured into the mold from above and filled from the bottom up entirely under the force of gravity. In die casting, molten metal is injected into a mold under considerable pressure (between 1500 and 25,400 psi). This pressure is maintained until the casting solidifies.

  The high pressure filling of the mold in die casting allows for very rapid injection of the alloy which, combined with the highly automated nature of the process, allows for very high production rates while reducing the possibility of human error. Since gravity die casting relies on gravity to fill the mold, the process is slower and therefore less suitable for long production runs.

  The need to deliver molten metal under pressure into the mold in a highly automated process makes die casting setup complex and expensive. Tooling costs can also vary considerably, as pressure die casting molds need to be made of hardened steel and often require internal cooling channels to dissipate heat quickly. In contrast, gravity casting molds can be made of cast iron, and the lower cycle rate prevents heat buildup in the mold and allows the casting to cool quickly. Gravity die casting also has the advantage that sand cores can be used to create internal voids that cannot be achieved with pressure die casting.

  The casting quality achieved by each process is different. Rapid filling of molds under pressure enables die casting to achieve very precise and complex castings that are dimensionally accurate and have thinner wall thicknesses. The as-cast surface can be very smooth, thereby reducing secondary processing. While gravity die casting cannot meet these attributes, the slower velocity of the molten metal entering the mold does mean that folding and turbulence occur less, so less air is trapped in the casting. This gives gravity die casting an advantage where subsequent heat treatment of the casting is required, although pressure die casting is stronger in the as-cast state.

  Obviously, die casting is much more expensive to set up and less flexible than gravity casting due to the complexity of the casting process. However, the potential for high productivity means that die casting is better suited for steady high-volume production runs where the unit cost of castings can be very low.

  The choice of casting process usually depends on a number of design specifications and the geometry of the part to be cast, as well as economic factors such as the production run. Sometimes gravity die casting is the right solution, and sometimes projects are better suited to pressure die casting. That's why we at Mingdao offer both options and many others, including sand casting, investment casting, lost foam casting and shell casting.