Maximising performance of thermoset composite materials, requires, amongst other things, an increase in the fibre to resin ratio and removal of all air voids. This can be achieved by subjecting the material to elevated pressures and temperatures. As described in the vacuum bagging section, some pressure can be exerted by applying a vacuum to a sealed bag containing the resin/fibre layup. However, to achieve three dimensional, uniform pressures of greater than 1 bar, additional external pressure is required. The most controllable method of achieving this for an infinite variety of different shapes and sizes is by applying a compressed gas into a pressure vessel containing the composite layup. In practice, this is achieved in an autoclave.
Autoclaves - A Brief History
Autoclaves have been used in industry for many decades. As technology has progressed so has autoclave design, initially from basic riveted steam heated vessels to vessels fabricated utilising the latest welding techniques with highly sophisticated computerised control systems.
The industries that make use of autoclaves have also evolved over the years. Initially used in the textile, timber, food, sterilising and rubber industries, autoclaves are now essential items in the advanced composites and investment casting industries.
For use in the production of advanced composite materials, a hot atmosphere autoclave has to achieve the following criteria:
Safety is of paramount importance when working with autoclaves. They are generally situated in an area where personnel are present. To put it in perspective, standing in front of a 1m diameter autoclave pressurised to just 0.5bar (typical working pressures are 5-7bar), is like standing underneath a 4 ton weight.
The UK Health and Safety Executive has produced a Guidance Note PM73 'Safety at Autoclaves' so that safety devices are fitted to:
Due to the critical nature of these devices, all must be designed to ‘fail to safety’. They can be of mechanical or electrical design, but any electrical devices should be ‘dual circuit cross monitored’.
Temperature control is of critical importance where resin systems are involved and the ability to achieve a uniform temperature inside the autoclave, within the tight tolerances stipulated by the aerospace industry, is of paramount importance. There are several systems employed by autoclave manufacturers to achieve this, these fall into two main categories, direct and indirect heating systems. Indirect heating systems have the heat source outside the autoclave and transfer heat into the working envelope by means of a heat exchanger. Direct heating systems have their heat source within the autoclave and aim to maximise the heat transfer from the elements to the pressure medium.
Electrically heated and gas heated systems are available. The choice of heating system is influenced by various factors such as controllability, cleanliness, efficiency, maintenance requirements and running costs. When considering running costs, it is important to evaluate the system as a whole.
Control systems are designed to manage cure cycle temperature strategies in accordance with the desired criteria. Temperature profiles are generally made up of a series of heating gradients, dwells and cooling gradients. The graph below shows a typical temperature profile specification.
Autoclave systems are designed to allow the user to set the internal pressure conditions to the required level at any time during the cure cycle. As in the temperature control, this generally takes the form of a series of pressurisation gradients, dwells and de-pressurisation gradients. Accurate control is achieved through the use of modulating valves on the inlet and exhaust pipework systems of the autoclave.
Safety is of paramount importance, when working with pressurised systems. Strict design codes (eg. PD5500, ASME) are specified to ensure the necessary safety margins are in place. All new autoclaves undergo a hydrotest, pressurising the vessel with water, up to a pressure of 1.5 times the stipulated maximum working pressure, prior to gaining certification for use. In addition to this all vessels are fitted with a relief valve, typically set to 10% above the stipulated maximum working pressure, which will release in the event of a pressure overload. This safety valve is a mechanical device ensuring that a dangerous situation is avoided even if the electronic control system devices fail.
The bagged composite lay-up arrives at the autoclave with a vacuum already drawn. The bag is then connected to the autoclave vacuum system via flexible umbilical hoses fitted inside the autoclave. During the cure cycle the vacuum level in each bagged component is monitored and in the event that a leak or burst condition occurs, the offending bag is automatically isolated from the rest of the vacuum system, thus preventing positive pressure from entering other bags via a common manifold. During the cure process, volatile substances are created within the bag. These are drawn away by the vacuum system and filtered out by a resin trap fitted in the main vacuum draw line. Systems are available with the facility to vary the level of vacuum drawn within the bag. This is of particular relevance when honeycomb structures are produced.
Computerised Process Control, Monitoring and Data Logging
The most significant advances in autoclave technology over the last 10 years have been in the field of electronic control systems. The sophisticated systems of today are a far cry from the manually controlled systems of yesteryear. Due to ever increasing pressures on productivity, there is a constant drive to achieve reduced cure cycle times, while maintaining product quality and repeatability. The majority of today’s autoclaves, supplied to the advanced composites industry, are supplied with a PC and SCADA software. These enable the user to monitor the cure parameters in real-time both for data-logging purposes and also for cure optimisation.
It has been said for years that with the development of modern thermoplastic composites and resin transfer moulding manufacturing processes that the future for composite curing autoclaves is uncertain.
The quest however for lighter, faster and more agile fighter aircraft, larger passenger aircraft and increasingly higher performance motor sport vehicles, has lead to an increase both in the size and sophistication of the modern day autoclave in increasing numbers. Whilst the search is still on for alternative curing techniques and increased performance from RTM manufactured components, the autoclave still serves as the every day workhorse for the world’s aerospace and motor sport industries and would appear to still have an exciting future.
Published courtesy of Robert Pickard, LBBC