Low-pressure casting is a casting method that uses low pressure (max. 2 bar) to fill molds with molten aluminum alloys. In this process, a pressure-tight furnace with a molten alloy is positioned under the casting, and molten metal is pushed through the riser tube (stalk) up and into the mold. The pressure in the furnace is applied continuously and can be adjusted to control the pouring speed and is maintained throughout the whole solidification process. Once the casting and the sprue have fully solidified, the pressure is released. The rest of the molten aluminum from the gating system runs down to the furnace. After the necessary solidification time, the casting, now sufficiently hardened, is removed from the mold. Then, the whole process can recommence and be continued until the furnace is empty.
Low-pressure casting allows for precise pouring process control and complete or partial elimination of risers (feeders), thus largely improving the yield. Low pressure pouring of molten aluminum alloys into molds reduces oxide and porosity formation, ensuring uniformity of the metal throughout the entire casting. Low-pressure casting guarantees high density and strength values, as well as high dimensional accuracy. Even though this method is more suitable for rather simple, symmetrical castings, more complex, intricate geometries can also be achieved through appropriate parting line designs in metal cores or by using sand cores. Thanks to its automation potential, the low-pressure casting process is well suited for high-volume production.
From the metallurgical point of view, low-pressure casting produces sound castings. The use of a controlled, turbulence-free pouring process almost completely prevents air from being trapped. Additionally, direct feeding of metal in the direction of solidification reduces the negative effects of shrinkage. These two factors can significantly reduce porosity in the casting. That is why low-pressure castings that undergo heat-treatment have greatly increased properties. The use of the vented metal die allows for controlled solidification, which additionally improves the mechanical properties of the casting.
The automotive and machine industries have been reliant on high-quality low-pressure aluminum castings for years. Thanks to low-pressure casting, modern foundries have greatly reduced their casting cycle times in recent years thus making low-pressure casting more and more economical.
Low-pressure casting is a good choice for components such as engine blocks, housings, gearbox components, and suspension components. Industries such as electronics, machinery, agriculture, and rail also benefit from lower machining costs, good performance, and metallurgical quality of low-pressure castings. The perfect electrical and thermal conductivity of aluminum makes it a highly desirable choice for many industries.
The difference between gravity casting and low-pressure casting is that in gravity casting, gravity is employed to fill the mold with metal. Molten metal is poured into the sprue located in the cope, which introduces the metal to the gating system together with runners and gates in the drag. The pouring speed depends on the shape and cross-section of the gating system and it decreases as the mold is filled.
In the case of low-pressure casting, the sprue is located in the drag and it often serves as the sprue and the only gate. The metal fills the mold cavity under a small pressure. Thanks to the capability to control the pressure, the whole pouring process can be fully controlled in terms of speed and it is possible to maintain the appropriate pressure during solidification to control feeding.