The heat of the moment

What do a dental surgery, a waste processing plant, a research laboratory and a large aircraft manufacturing factory have in common? The answer is – the autoclave.
Aerospace manufacturers have been using the autoclave for decades. However a careful look at its history, design and evolution shows that they are much more than simply ‘pressure cookers'.

Autoclaves in the aircraft industry need to be able to reach air temperatures typically in excess of 240°C within 45 minutes, and to an accuracy typically of ±2.5°C (though this can be as low as ±0.9°C); and pressures up to 15bar, though more typically 10bar (determined by the type of resin system being used) to an accuracy of ±0.1 bar.

In partnership with AIC Group, AeroThermal is currently designing and manufacturing a state-of-the-art 18ft diameter autoclave, the first of its size in Scotland, to enable UTC Aerospace Systems to maintain, repair and overhaul larger nacelles systems fitted to the next generation of wide-bodied aircraft.

“Autoclaves at heart are simple machines,” begins Dorset-based AeroThermal Group's managing director, Ian Toll. “But the devil is in the detail. Making an autoclave 18ft in diameter or 40m long is a wholly different game to creating a machine for use in a dental surgery.

“For a start, you will need to decide the number of thermocouples and how these are to be scanned and recorded, or included in the control loops; and where they should be positioned. This includes both thermocouples as part of the structure of the autoclave as well as those attached to the tool, and how they should be attached to the tool.

“You will need to consider the thermodynamics of the airflow through the chamber, to ensure even heating. There's no point in one half of the tool being one temperature and the other four or five degrees higher. And that also applies to the degree of vacuum which will dictate solvent flow, which is also effected by the choice of resin.

“Some components will take more vacuum than others. So a key design feature is how many vacuum lines are needed for the tool, and where they should be located. Autoclaves can have up to four different vacuum headers, which apply different levels of vacuum to different parts, say. There is also another control needed in case one of the vacuum bags splits.”

Control and uniformity

All these variables need to be calculated and incorporated into the design. Heating uniformly in a modern autoclave is a problem which has been overcome, largely due to the patented gas fired radiant tube heated autoclaves. Heating can be carried out using oil (filled radiator), gas (fired burner), electricity or steam (filled radiator), and the criteria for choosing each of these includes cost and length of machine (e.g. it could be too short for a radiant gas tube heating system).

Nowadays circumferential airflow is also used in larger autoclaves. Toll continues: “Consider the 120ft wing skins of an A380, which is very wide at one end and narrow at the other and highly curved. A normal autoclave would have a fan at one end, so sucking the air from the door end and returning, with the fan pushing the air back along the length of the autoclave in a duct, over the heaters then back over the tool. This is axial flow.

“But it would be very difficult to achieve the tolerances needed over 120ft as the fan would have to be enormous. So instead of sending the air 40m lengthwise, let's split the machine into nine, 4m segments, each with a fan to run the air around the circumference.

“The heaters are in the duct and the fan sends the air over the heaters, enabling different temperatures along the length of the tool. The fan outlets are also angled in this configuration to facilitate and promote mixing of temperature and heat transfer throughout the length of the machine through forced convection.”

At the other end of the process, the chamber (and tool/composites part) needs to be cooled. This can be achieved with: a water cooled radiator, but at very high temperatures the water will flash into steam which can create a choking flow and the steam will jam up the pipe work; an oil heated radiator could be used as a cooling mechanism via heat exchanger, but this would be a slow process.

The environment within the autoclave is also important. Nitrogen as an inert gas is often used instead of air. This is due to safety as combining oxygen, pressure and heat can be an explosive combination at worst, and at best could destroy the inner liner of the autoclave, the mould tool and the component.

Autoclaves have very specific designs, depending on the application. Toll comments: “It's fairly straight forward to manufacture standardised machines to a set volume, diameter and to a given pressure and temperature. What really interests me are the unique applications, with bespoke requirements. That's where the real challenge comes. We have even built a nitrogen autoclave for packaging live bombs under pressure!”

Particular attention also needs to be paid to the strength and material of the pressure vessel shell. Thickness varies but can be 60mm or more, depending on the host country's regulations, the diameter of the machine and operating temperature and pressure.

Composites under pressure

The main use of autoclaves in the aerospace industry is for the consolidation and curing of composites or the age-forming of aluminium alloys. There are many ways to cure composite, but the most effective methods, in simplistic terms, require temperature to kick off the catalyst and pressure to squeeze the air from the layers so that the laminates bond strongly together, without resin voids or micro porosity.

This is what produces the strength with light weight which is so highly sought after by advanced engineering industries. Otherwise the fibres can shear, reducing strength or creating micro creases within the material.

The autoclave is uniquely placed to do this, as it operates with a vacuum, which squeezes the laminates on the tool together, ensuring uniform spreading of adhesive and/or resin. And when put under pressure greater than that achieved under vacuum alone, also minimises this micro porosity - a risk as the prepreg resin's solvent boils off. It also removes moisture and phenol carryover from phenolic resins.

Much has been made of the imminent demise of autoclaves. Certainly Toll remembers being lectured when he started in the industry in 1982 that the autoclave would be dead in five years. And yet the industry keeps using them more and more.

Autoclave designers are now looking to ever more sophisticated control of all the variables within the pressure vessel. Toll sums it up: “It is all about quality control. Until out of autoclave curing can be as efficient and effective as the pressure vessel, autoclaves are here to stay.”

www.aerothermalgroup.com