Pressure vs. Gravity
An important factor in classifying aluminum casting processes is whether the molten metal flows into the mold by gravity or whether it is forced into the mold cavity by pressure:
- Injection molding requires reusable molds and is commonly referred to as high pressure molding.
- The low pressure casting process is considered a variant of gravity casting in permanent molds .
Low Pressure Casting
Low pressure casting process
Low pressure casting is a well-known casting process. In particular, most aluminum wheels are made using low pressure casting. This method applies:
- pressure about 2 bar.
- Filling of the casting mold is carried out by supplying compressed air to the holding furnace.
- The melt is fed from bottom to top through a return pipe, which is immersed in the depth of the melt.
- This ensures smooth filling of the mold and clean melt.
- After the casting mold is filled, solidification of the metal begins on the opposite side of the profitable part.
- By adjusting the air pressure in the holding furnace, the mold is completely filled.
- The casting cycle is quite long due to the fact that the casting is connected through a return pipe with a large volume of melt in the furnace during the entire solidification time .
Low pressure casting machine
The low pressure injection molding machine is oriented vertically, resulting in a horizontal parting line (Fig. 2). The molten metal is contained in a sealed vessel, which is then compressed by a gas (usually air) so that the metal is forced “up” into the mold cavity. This is beneficial as it results in controlled form completion. This process is mainly used to produce automobile wheels. It is interesting to note that each wheel is x-rayed as these are critical parts .
Fig. 2 – View of the Low Pressure Die Casting Process 
Low pressure casting machine includes (Fig. 3):
- sealed holding oven
- compressed air supply and control equipment
- hydraulic manipulator for installation and removal of casting molds
- typically, a low pressure injection molding machine works with one mold.
Fig. 3 – Aluminum alloy casting machine
under low pressure. Below is the holding furnace 
Low pressure casting technology
In this process, permanent molds are installed over a sealed oven. The feed pipe exits the mold cavity into the molten metal below. Due to the increase in pressure in the furnace, the metal is forced through the pipe into the cavity of the mold (Fig. 4). Once the metal has hardened, the pressure is released, the mold is opened and the casting is removed, preparing to repeat the cycle.
Main technological parameters
Process parameters include the speed of application of pressure, which regulates the filling of the mold pressure, which is relatively unimportant once solidification has begun temperature gradients that are necessary to ensure directional solidification.
As in a conventional permanent mold, these gradients are established by the selection and controlled thickness of the mold coatings and the selective cooling of the mold sections.
Most low-pressure castings are made using only one metal entry point, and the risers normally needed to prevent internal shrinkage voids are not typically used. Therefore, the gross to net weight ratio is low and the trimming and finishing operations associated with the gates are kept to a minimum.
The low-pressure casting cycle is driven by the solidification of the metal at the junction of the filler tube and the mold cavity. While the counter-gravity flow of metal into the mold cavity is at rest, the process poses a risk of contamination by inclusions.
When the mold is opened and the casting is removed, the vacuum seal that existed at the liquid-solid interface is broken and the molten metal remaining in the tube falls to the level of the furnace metal. Cyclic flow of metal in a vertical direction in a fill tube can result in the accumulation of oxides on the internal surfaces of the fill tube, regardless of whether the tube is ceramic or coated metal. To minimize this, the system is maintained at back pressure to keep the molten metal at an elevated level in the fill pipe at all times. In this case, it is possible to replenish the metal in the furnace at each cycle, and not periodically when the metal in the furnace is almost empty.
Filtration of the metal at the point of entry into the mold is usually used to prevent contamination of the casting by inclusions. Filtration may consist of steel mesh or ceramic filters.
Fig. 4 – Low-pressure permanent mold 
Aluminium low pressure castings
- Previously, most low pressure casting was limited to radially symmetrical castings. Thus, almost all car wheels are cast with this method (Fig. 5, 6).
- Nowadays, low pressure casting is used to produce many asymmetrical parts (Fig. 7).
Fig. 6 – Alloy A356.0 alloy automotive wheels produced by low-pressure casting