LOW PRESSURE CASTING

The low pressure casting is a counter-gravity casting method, when, instead of pouring the molten metal into the casting mould and allowing gravity to be the force that distributes the liquid material through the mould, air or gas pressure is used to force the metal through the gating system and the metal casting’s cavity. The molten metal enters the mould from below under a relative low pressure, bottom gating is used. This process can be used to cast high-quality manufactured parts. In low pressure casting, the mould is set up above a sealed airtight chamber that contains a crucible holding molten metal. A refractory tube, the fill tube, extends from the entrance of the gating system of the mould down into the molten material. When the mould is prepared and ready for pouring the casting, the metal flow is accomplished by pressurizing the chamber. This creates air pressure on the surface of the liquid metal that, in turn, forces molten material up the fill tube and throughout the mould. The metal is moved into the mould by an air pressure differential between the chamber and the mould at a rate enabling the air to escape through joints and vents in the mould. The molten metal slowly fills the mould, reducing turbulence. The rate of metal flow is controlled by the rate of pressurization of the chamber; air pressures in the range 20–100 kPa are used. The upward feeding is maintained from the molten metal even after filling the mould, which allows the metal to be fed into the casting during solidification. The metal in the fill tube acts in this way as the feeder. On releasing the pressure, the surplus liquid metal returns to the crucible and all that remains is a small projection on the casting. Such a system involves no runners or feeder heads in the accepted sense, so that very favourable casting yields are achieved (approaching 100%). The operational cycle of the casting is necessarily slower than that for gravity or pressure die casting, owing to the need to maintain the pressure until solidification of the casting is complete.

Applicability of this casting method for various metals and alloys is restricted only by the mould used. Metallic permanent moulds are most widely utilized for low-pressure casting of non-ferrous alloys, but refractory moulds can be used, as well. Ceramic moulds for the production of investment castings, common aggregate and shell mould are also used. Steel castings are often cast in graphite moulds, using this process. To produce internal cavities, the aggregate or shell cores can be used, not only metal ones as in the case of the high pressure casting. The aggregated or shell cores usually require a refractory coating to prevent metal penetration under pressure.