Low-pressure Casting
Low-pressure casting is generally powered by compressed air, either air or inert gas. The air passes into the closed vessel (crucible) and acts on the surface of the molten metal alloy that maintains a specific pouring temperature. And it causes a pressure difference between the sealed vessel and the mold’s cavity. And under this low pressure of 0.01-0.05MPa, the metal liquid in the closed vessel will flow along the liquid-raising channel, pass through the mold gate, and fill the cavity smoothly.
After the metal liquid fills the cavity, we need to increase the air pressure to make the molten metal cool, crystallize and solidify from top to bottom. When the solidification of the casting part gets finished, the air pressure will be withdrawn, and the liquid that has not yet solidified in the channel and sprue will fall back to the closed vessel by its gravity.
The procedure is a cycle, and the process’s pressure, time, speed, and temperature are all controllable.
High-pressure casting is often referred to as die casting. It is the process of pouring the metal liquid into the pressure chamber, filling the mold cavity at high speed, then crystallizing and solidifying under high pressure, and finally forming the casting blanks.
High pressure and high speed are the main features of high-pressure casting(die-casting), and the flow speed during filling can be up to 30-80mm/s or even higher. The metal liquid solidifies under pressure as high as 40-120MPa, and the time for the metal liquid to fill the die cavity is extremely short, usually less than 1 second.
Die casting has the advantages of good surface finish, dimensional stability, and direct molding of thin-walled structures. However, it also has several disadvantages, like more casting porosity, unsuiting heat treatment, and high operating costs.
Comparison of Low-Pressure Casting & High-Pressure Casting(Die Casting)
The following table summarizes some of the characteristics of high-pressure casting(die casting) and low-pressure casting, comparing and analyzing the advantages and disadvantages of the two processes.
| Low-Pressure Casting | High-Pressure Casting(Die Casting) |
| The pressure is low, generally less than 0.08 MPa, and can reach 0.15 MPa in exceptional cases. | The pressure is higher and can reach hundreds of MPa. |
| The flow speed of the metal liquid is slow, generally 150mm/s in the casting state. And the scouring of the cavity by the metal liquid is small. | The flow speed of the metal liquid is fast, which can reach 60m/s and up to about 120m/s. The scouring of the cavity by the metal liquid is large. |
| The flow of the metal fluid is smooth. | The flow of the metal fluid is relatively unstable. |
| The material required for the mold cavity is relatively low. It can be metal, sand, or other materials. | The material required for the mold cavity is high, and it can only use metal dies. |
| It is possible to produce castings with more complex cavities. | Currently, die casting can only produce castings with relatively simple cavities. |
| It can produce small and medium-sized castings as well as larger ones. | It is usually only suitable for the production of small and medium-sized castings. |
| No porosity inside the casting. | There may be porosity inside the casting, and a particular process is required for castings with airtightness requirements. |
| The castings can be heat treated. | It cannot be fully heat treated and strengthened. |
| The surface smoothness of the casting is relatively average. | The surface of the casting is relatively smooth. |
| The casting cycle is relatively long, and the production speed is average. | The casting cycle is relatively short, and the production speed is relatively fast. |
| The finished product rate is relatively high, about 80-95%. | The finished product rate is relatively low, about 60-90%. |
| The operating cost is low. And the low-pressure casting can be applied to situations where the quantity is not very large. | The equipment running cost is higher. And the high-pressure casting(die casting) is more suitable for high-volume order production. |
The above table shows the apparent advantages and disadvantages of both low-pressure casting and high-pressure casting/die-casting. And we need to choose the production process suitable for our products according to our needs. Design capability, application scenario, capital budget, product quantity, equipment capability, engineering development capability, and subsequent processing capability are all factors that we should consider to take advantage of the process and compensate for the disadvantages.
Conclusion
Therefore, choosing the process that meets our actual situation and design requirements is the key.
