As I mentioned in my last blog, curing in the simplest case, happens under standard atmospheric conditions. However, one can accelerate the curing process by applying pressure. Vacuum bag autoclaving is one of the common processes for curing prepreg plies. Dry or wet layup typically precedes vacuum bag autoclaving.
The semi-finished composite along with the tool into which it is laid out (mold for the component part) is placed in a sealed transparent film called vacuum bag. This bag is made of fabric, nylon, rubberized sheet or plastic. There are additional components called bleeder assemblies that allow removal of any gases generated and plies to overflow or bleed as they get stretched and consolidated during the curing process. The air inside the vacuum bag is removed using vacuum pump.
The vacuum bag is then placed into autoclaves for curing. Autoclaves are essentially giant pressure cookers. The items that need to be cured are first placed into the chamber and sealed. The air is removed using a vacuum pump. The chamber is pressurized further. The most controllable method to increase the pressure for components of different shapes and sizes is to apply a compressed gas into the chamber. Steam is gas commonly used for this purpose. The temperature is controlled by direct or indirect heating systems. Direct heating systems have the heat source inside the autoclave chamber. The aim is to maximize the transfer of heat between this source and pressurized gas inside the chamber. The heat source in an indirect heating system is outside the chamber. A heat exchanger transfers the heat into the chamber.
An autoclave is also often found in many laboratories to sterilize glassware and equipment. More on its use for sterilization in later blogs.
Acknowledgement: Arun Nallainathan contributed to the contents of this blog.