Permanent Mold Casting Process
The permanent mold casting process, also know as the gravity die casting process, is divided into six different steps:
Preparation of the Mold
First, the mold to be used in the casting process is preheated to between 150°C (300°F) and 260°C (500°F). This pre-heating allows for a better metal flow and also reduces defects caused during the casting process.
Assembly of the Mold
After the mold has been pre-heated, a ceramic coating s applied to the mold cavities. The coating allows for an easier part removal as well as aid in prolonging the life of the mold.
In the gravity die casting process, the mold used consist of at least two parts. This consists of the two mold halves and, if any, the cores used to facilitate more complex castings.
The cores used in the process are typically made from iron or steel, but in some cases expandable sand cores are also used.
This step involves the insertion of the cores and clamping the two mold halves together.
This step in the permanent mold casting process involves the molten metal being poured into the mold. This is done at a slow rate from a ladle into the mold through a sprue (which is located at the top of the mold).
The molten metal now flows through the runner system into the mold, filling the cavities.
During this step in the process, the molten metal poured in the previous step is allowed to cool and solidify in the shape of the mold.
Opening of the Mold
Once the molten metal has cooled and solidified, the two halves of the mold is opened and the casting is removed.
During the fourth step of the gravity die casting process, the sprues and runners used for pouring and regulating the flow of the molten metal, solidifies like the casting does, and needs to be removed.
Trimming involves the the excess material (the sprues and runners) being cut away from the casting.
The permanent mould casting process has also evolved and variations of the techniques has been developed in order to accommodate very specific applications. Some of these variations include:
Low Pressure Permanent Mold Casting
Instead of pouring the molten metal, the metal is forced into the mold by using low pressure air. This technique allows for the mold to remain filled and aids in reducing shrinkage caused during the cooling process. This techniques also allows for the molding of finer details and thinner cast walls.
Vacuum Permanent Mold Casting
Here, vacuum pressure is applied in order to fill the mold. Very much like the low pressure technique, vacuum casting allows thinner walls, finer details and better mechanical properties.
As seen in the conventional process, the molten metal is poured into the mold and allowed to cool and solidify at the cavity’s surface. When the amount of solidified material is equal to that of the desired wall thickness, the remaining material which has not yet solidified, is poured out of the mold. This technique is commonly used to cast hollow parts without making use of cores.
Watch: The Permanent Mould Casting Process
Back to Main Page: Permanent Mold (Gravity) Casting.
Further Suggested Reading:
- Permanent Mold Casting Tolerances
- Surface Finishes for Permanent Mold Castings
- Metals that can be Permanent Mold Casted
- The Limitations of Casting Using Permanent Mold Processes