What is Pressure Casting?
Pressure Casting (also known as low pressure casting process) is a type of permanent mold casting process in which the liquid metal is forced to move in the mold cavity under the application of air pressure.
Instead of allowing the gravity to distribute the molten metal, the low pressure air is used to force the molten metal in the mold cavity.
Low Pressure Casting Process (Steps)
- The very first step in pressure casting process is similar to that of the permanent mold casting process.
- First of all, the two blocks are accurately machined and they are made in such a way that they can be opened and closed for the removal of the final casting.
- The gating system is also made in the mould itself.
- The gating system made in the mold is such that the molten metal flows from bottom to top instead of top to bottom (see above image).
- In pressure casting, the position of the mold is kept above the liquid metal.
- The refractory tube is arranged as shown in the diagram which connects the mold and liquid metal.
- The refractory tube is kept deeper in the molten metal so that clean molten metal is pushed to the mould cavity.
- The chamber in which the liquid metal is kept is made air tight so that it prevents the leakage of air as well as it prevents the pressure drop.
- When the chamber, gating system, mold, etc are arranged, the air pressure is applied to the chamber.
- Pressure casting (or low pressure casting) uses a low pressure gas (between 20 kPa to 100 kPa) to push the molten metal into the mold cavity.
- Because of this, the upper surface of the molten metal experiences the pressure, which forces the molten metal to flow upward in the refractory tube.
- Finally the molten metal reaches the mold cavity and its solidification begins.
- It is necessary to maintain the constant air pressure until the metal solidifies.
- Because of the constant pressure during the filling of molten metal, there is a low turbulence and it minimizes gas porosity.
- After the solidification, the final casting is obtained and it is removed by opening the mold.
- The pressure used in pressure casting is generally low. Hence this process is also known as low pressure casting process.
Advantages of Pressure Casting
The advantages of pressure casting process are mentioned below.
- Good surface finish can be obtained by using a pressure casting process.
- The casted products have good dimensional accuracy.
- There is no wastage of molten metal as there is no runner and riser used in this process. The excess molten metal falls back to the ladle which can be reused.
- By using a pressure casting process, higher productivity rate can be achieved.
- Casted components with superior mechanical properties can be manufactured.
- The effect of oxidation of metals is highly reduced by using a pressure casting process.
- Pressure Casting is highly suitable for manufacturing a large batch of components.
Disadvantages of Pressure Casting
The disadvantages of pressure casting process are mentioned below.
- Preparing a metal mold for complicated geometry is difficult.
- The equipment and setup cost is high.
- This process is not beneficial for small runs.
Applications of Pressure Casting
The applications of pressure casting process are mentioned below.
- Pressure casting is applicable for manufacturing the castings which requires higher dimensional accuracy.
- Steel metals are casted in graphite molds by using this process.
- Low pressure casting is generally used for aluminum castings, magnesium castings and for some copper alloys.