Permanent mold casting is a metal casting process using reusable molds, commonly made of high melting point metals. The process is used for mass production or design duplication, and can use gravity, vacuum or pressure to introduce molten metal into the mold. Slush casting is a variation where the outer edges of the mold harden before the center is filled.
Permanent mold casting is a metal casting process using reusable or permanent molds. The permanent mold casting process is used in a variety of applications where a design will see mass production or design duplication. While the process is more expensive than other forms of metal casting, it is considered an affordable means of replicating parts for most major industries.
In most cases, permanent mold casting is done using a two-piece mold. These molds are usually constructed of high melting point metals, such as steel, graphite, bronze or cast iron. The two parts of the mold fit together perfectly and have an opening at the top for the molten metal to enter. After cooling, the two mold pieces are separated and the molten metal part is recovered for finishing.
To start the process, the mold is preheated. This step removes any moisture that may be present in the mold and prevents the molten metal from cooling too quickly. Preheating the permanent mold also prevents thermal expansion damage to the mold when molten metal is introduced.
The permanent mold casting process employs several means to introduce molten metals into the mold. The most common method is to use gravity to simply pour the liquid metal into the heated mold. This method is the least expensive method of permanent mold casting because it requires less equipment, but it is not suitable for situations where the liquid metal may not reach all areas of the mold.
In situations where the permanent mold has fine details that need to be filled with molten metal, vacuum or pressure assisted metal introduction may be required. Low pressure is used to force the molten metal into the mold in the case of pressure permanent shell casting. For vacuum-assisted permanent mold casting, air is pulled out of the mold, creating a low-pressure vacuum that draws the metal into the mold. In both cases, the pressurization causes the molten metal to fill in the small spaces and fine details that could otherwise mar a casting.
Another variation on the permanent mold casting process is the slush casting method. In this method, molten metal is poured into the mold and allowed to harden around the outer edges like a shell. Once the molten metal begins to harden, the liquid metal is poured into the centre, leaving a hollow casting suitable for decorations and ornamental products.
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