Pressure Die Casting
Pressure die casting is a means of mass-producing low temperature metallic components with a high degree of precision and repeatability. Unlike gravity die casting, the process is automated and the liquid metal or alloy is injected under high force into a hardened steel tool. This means pressure die casting can be performed at a low per-unit cost in longer production runs which soon makes up for higher initial tooling costs.
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Components produced by pressure die casting are known for their exceptional dimensional accuracy and smooth surface finish. Walls can be cast with thicknesses below 3mm, and surface roughness below 1.5 Ra, which also reduces secondary machining costs. Due to the automated nature of the process, there is less design flexibility than with the gravity process. Because the metal or alloy is injected under high pressure – generally over 10,000psi – the mould is filled rapidly, and solidification results in components with high fatigue strength and a fine grain structure. Injecting the metal under pressure also has the advantage of minimising metal loss to spillage and speeding up production times. The metal dies used need to be more resilient than those used in gravity die casting, partly due to the pressures involved but mainly due to the potential for much higher throughput. Because of this higher production requirement, the wear, and the associated heat produced, the tooling usually incorporates a water cooling system and this can mean a higher initial outlay. However, this investment has to be considered against the extended tool life, the process repeatability and the ease with which it can be left to automation.
There are a range of factors to take into consideration when making a decision regarding which casting process is the most suitable. These can include quality requirements such as surface finish, complexity, mechanical properties and integrity, as well as production considerations such as production rate, lead time, and process flexibility and other commercial considerations.
Pressure die casting is only suitable for low temperature metals such as aluminium and some copper based alloys, to produce smaller components in large volumes. Due to the automated nature of the process, the production lead times in comparison to other casting processes are low, although initial tooling lead times can be slightly longer. Due to the relatively high initial tooling costs and potential difficulties associated with modifying automated hardened tooling, only stable designs with higher volume requirements should be considered for pressure die casting.
Pressure die casting also allows for the casting of magnesium alloys, which are being used more and more in the car manufacturing industry.
We use computer-aided design (CAD) technology to produce a 3D model of your component. After successful flow simulations have been achieved, the 3D model is used to produce the two die halves of the tooling. The tooling also incorporates an “ejector” system, used to free the component from the tooling between the casting cycles.
The die halves, made from hardened steel, are secured to the casting machine, one fixed and one on the moving hydraulic ram, cleaned and lubricated with release agent, before being closed under hydraulic force. The mould will remain clamped as the liquid metal is injected. Molten metal decanted from a defined source on or close to the machine, often by robotic arm and is injected under high pressure into the cavity. The metal is allowed to cool and solidify within the mould to take on its shape and form. The pressure is maintained throughout the cooling period, the length of which depends on the thermodynamic properties of the metal used. The tool is then opened under the machine cycle and the part ejected. A certain amount of force is required for ejection, as parts can shrink and stick to the mould during cooling. This can result in small ejector pin marks, this should be considered at the design stage in order to ensure they are either removed by post processing, or located in a none functional area.