Pressure casting at Hallam Castings

Hallam has retained its core expertise in pressure casting and produces around 20,000 zinc and aluminium castings per week from 10 die casting machines capable of making aluminium castings weighing up to 5 kg.

There are several ways to make aluminium castings; Hallam Castings specialises in cold chamber high pressure die casting. This is the process of injecting molten aluminium under extreme pressure into a mould which forms the shape of the component or product required. Tolerances of + / - 0.05 mm can be achieved.

High pressure casting tools must be made to close tolerances with finer surface finishes and withstand extreme pressures and heat. These types of tools are designed to reproduce parts in high volumes with fast cycle times at an economical unit price.

High pressure die casting parts, without careful mould design, are more susceptible to porosity than sand or gravity processes.The die mould has one open end where the metal is injected under pressure; when the two halves of the die are closed under extreme tonnage the mould cavity is still full of air. The majority of the air has to be displaced from the die as the molten metal is injected. This can be achieved by placing vents at strategic places within the die to release the air. Hallam uses dynamic CAD models to design the tool to ensure that the air is not trapped in the mould. The CAD models also allow Hallam to calculate the amount of draft required for the aluminium moulding to be released easily from the die. Typically draft angles of between 1 and 2 % are applied.

Hallam Castings' knowledge of porosity has been enhanced by research undertaken in this area by the university of Sheffield at Hallam's premises.

As the core and cavity retreat with the applied draft, they separate from the zinc or aluminium casting; if no draft is applied, the side walls of the casting, (marked with red arrows), stay tight to the side walls of the core and cavity. This creates friction between the die and the aluminium moulding and can potentially damage the die and the part.

It is important to involve the supplier of the part to be cast at an early stage in the design process. Hallam is able to advise the component or product designer about potential pressure casting process issues.