Vacuum bagging techniques have been developed for fabricating a variety of components, but mainly for complex shapes, double contours and relatively large components.

The process is particularly suited to the moulding of low cost components which are too large and/or complex to be pressurised by other means. The technique utilises a flexible sealed bag under which a vacuum is drawn, thus exposing the laminate in the mould below to atmospheric pressure (1 atmosphere = 14 psi). The assembly is then heated in an oven to promote flow and cure the resin. This method requires low cost equipment and tooling, and is capable of producing advanced composite components to a quality which is acceptable for many applications.

Cure Temperature/Time

For each prepreg resin system there is a range of options for cure temperature/duration. There is also a minimum cure temperature. There will also be a corresponding cure time for each given cure temperature. The oven/autoclave, component and tooling, should all reach, and remain, above the given cure temperature throughout the specified cure cycle. Thermocouples are used to monitor the temperature of the component and tooling.

Heat Up Rate

The heat up rate dictates how quickly the component/tool is brought up to the cure temperature. This is governed by numerous factors, namel: matrix viscosity and reaction rate, thickness of laminate, and tool mass and conductivity. For highly reactive matrices and thick laminates, the heat up rate will be low in order to avoid exothermic reactions.

Cooling Rate

The cooling rate is controlled in order to avoid sudden temperature drops, which may induce high thermal stresses in the component.

Typical Vacuum bag processing cycle

• Apply full vacuum at room temperature, aiming for 982 mbar (29 inches Hg) vacuum pressure on the laminate.
• Heat to 65 ±2°C (149 ±3.5°F) at 1°C (1.8°F) per minute, or up to 3°C (5.5°F) per minute if the oven is capable of this ramp rate.
• Maintain at 65 ±2°C (149 ±3.5°F) under applied vacuum for 2 hours to ensure the laminate and mould tool reach an equilibrium temperature.
• Heat from 65 ±2°C (149 ±3.5°F) to 80 ±2°C (176 ±3.5°F) at 1°C (1.8°F) per minute.
• Maintain at 80 ±2°C (176 ±3.5°F) under applied vacuum for 5 hours to complete the cure.
• Cool to room temperature at 3°C (5.4°F) per minute maximum under vacuum.