vacuum casting

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Vacuum casting is a means of casting small metal parts or jewelry that have fine detail or for casting various plastic materials. A porous or vented mold is used and is placed on a table or container where vacuum is applied. The liquid to be cast will be driven into the mold by atmospheric pressure, while the vacuum will also remove trapped air that would otherwise impede the free flow of the liquid casting material. Vacuum casting is applied as an alternative to centrifugal casting of metals which is used in similar situations.

Vacuum pressure casting or VPC is a recent development in metal casting, whereby vacuum is used in combination with various gases under pressure to improve the quality of the casting and minimize porosity in the metal. Typically VPC casting machines consist of an upper and a lower chamber. The upper chamber or melting chamber housing the crucible, and the lower casting chamber housing the investment mould. Both chambers are connected via a small hole containing a stopper. A computer program governs the various cycles involved in the casting process. A typical casting cycle, would progress as follows;

1. The preheated investment mould is placed in the casting chamber which is then closed tight.
2. The metal casting granules are then placed into a graphite crucible in the melting chamber. The melting chamber is also closed with an airtight seal.
3. The air in the chambers is then evacuated
4. The chamber is then filled with an inert gas, such as Helium to prevent oxidation of the metal alloy during the melting stage.
5. The casting alloy is then heated up to the desired casting temperature
6. Upon reaching the desired temperature, the melting gas Helium is evacuated from the chamber, immediately after which the crucible stopper is lifted, allowing the molten alloy to flow under the force of gravity into the mould.
7. Immediately after the pour, the upper chamber is pressurised with Argon gas, which effectively pushes the molten alloy into the mould, ensuring that any shrinkage porosity is minimised.
8. Once the casting alloy has solidified in the mould, the chambers can be depressurised and opened, allowing for the removal of the casting.

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