Vacuum Casting speed/pressure - Projects & Prints

When it comes to vacuum casting, the primary consideration is the role that vacuum plays.

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Initially, if you vacuum the material prior to pouring it into the mold, you are performing a degassing process. This step is crucial in removing trapped air from the material.

Conversely, if you apply a vacuum to the mold after it has been filled with material, you effectively reduce the pressure on the casting inside. This reduction in pressure causes the air bubbles within the material to expand significantly, making them more buoyant. Consequently, they rise to the surface, burst, and are subsequently expelled by the pump.

However, if you maintain the mold under vacuum, the bubbles remain trapped inside, albeit at a lower pressure.

The ideal approach is to apply vacuum to the mold, and after the material is still in its liquid state, promptly release the vacuum. This technique generates approximately 14 lbs of pressure on the material, which can effectively eliminate any remaining bubbles by either compressing them to a minuscule size or dissolving them back into the mixture.

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This technique can be compared to pressure casting, where the material is simultaneously degassed and the remaining bubbles are compressed before reintroducing full atmospheric pressure.

However, there are a few important cautions to bear in mind:

First, evacuating a chamber takes significantly more time than simply applying 80 psi of pressure. It's essential to select materials that remain liquid long enough, allowing you adequate time to achieve vacuum and to hold it long enough for the air to escape before releasing the vacuum.

Because of these requirements, many urethane materials are not suitable, as they cure too quickly. Most vacuum casting processes utilize epoxy or polyester resins, which provide a minimum of 15 minutes of workable time.

Secondly, materials under vacuum can inflate to three or four times their original volume due to air expansion. This phenomenon can lead to resin spilling over the top of the mold, resulting in bubbles for an extended period. Therefore, when vacuum casting, it is vital to design your molds to accommodate this overflow and keep the resin contained at the gates, allowing trapped air to escape and the resin to flow back into the mold.

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