The material to be degassed is placed inside a vacuum chamber, the partial pressure within the chamber allows the gases in the material to escape so reducing the chance of imperfections in the cured item.
Vacuum degassing is a method employed to improve mould making and castings.
Less prone to imperfections due to desolved gases and trapped bubbles within the casting material while it is still in a liquid state.
Vacuum Degassing - Step by Step Guide
Vacuum degassing technical notes