Turn up the heat on repair

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Tom Lane, managing director of Heatcon Composite Systems (Europe), asks whether your repair and rework capability is being restricted by the lack of an autoclave?

A large number of composite repairs can be accomplished by a hot bonder, utilising vacuum pressure for material compaction. However, there are many instances for which the Structural Repair Manual (SRM) stipulates that a positive pressure, typically up to 50psi, must be used, as compaction of the material created by vacuum alone is insufficient for carrying out large area repairs on major components.

In such instances, up until recently the only alternative to utilising an autoclave was to combine vacuum with a ‘dead weight’, such as sandbags, in order to achieve the equivalent of the required positive pressure. As it is difficult to achieve pressure uniformity over the repair area, plus the fact that sandbags act as a huge ‘heat sink’, most repair stations without an autoclave would either decline the job or subcontract the repair to a facility with one.

Another, more practical alternative is now available since Heatcon Composite Systems introduced the RepairClave to complement its existing range of composite repair equipment.

Having been involved with the repair of aerospace composite structures for over thirty years, Heatcon was well aware of the desire of repair stations to have an autoclave on-site, but also aware of the near impossibility of having a capital equipment requisition signed off by management, for the purchase of one. The huge financial investment in an autoclave is a serious commitment for any organisation and only the largest of repair stations are likely to see a satisfactory return on the expenditure.

Keep it simple

Being primarily designed for the manufacture of parts, the autoclave could be considered to be over-engineered for repair applications and so the main design concept for the RepairClave was to keep it as simple as possible whilst still being capable of meeting all the operational parameters.

Being confident in all aspects of heat management and an in-depth experience with hot bonding techniques meant that the only real unknown feature to be covered was that of introducing the required positive pressure and being able to vent the vacuum bag at the appropriate stage of the cure.

The ‘well proven’ silicon rubber heater blankets, which could, if required be shaped to the contour of a specific part were easily capable of operating at the temperatures and pressures specified and so a suitable pressure vessel, with ‘through wall’ connectors for power cables, thermocouples and vacuum hoses was developed. The first experimental unit was sized to accommodate any Boeing 737 flight control, but subsequently variations of the RepairClave have been manufactured to a range of diameters and lengths.

The biggest advantage of using heater blankets to provide localised heat is that the total volume inside the pressure vessel does not need to be heated to the cure temperature and therefore the operational costs, as compared to that of a gas, electric, oil or steam operated autoclave are minimal. Another cost saving is that compressed air can be used as the pressure media, as the use of heater blankets allow the internal air temperature to be kept well below that of the cure. As the total airspace inside an autoclave is heated to operational cure temperature, for fire prevention reasons an inert gas such as nitrogen must be used for pressurisation once temperatures reach or exceed 250ºF/120ºC.

Small- and medium-sized autoclave operators would usually have this gas delivered in cylinders by an industrial gas supplier, whereas large installations sometimes have their own dedicated nitrogen generation plant. Either way, this is an expense and inconvenience which does not apply to the RepairClave.

Localised heat also means that just the repair area of the component is heated and therefore there is less likely to be the need for tooling which would be the case with an autoclave, in which the entire part is subjected to the cure temperature.

Temperature uniformity in an autoclave is achieved by a circulating fan forcing a large volume of the gas used for pressurisation, over the part at high velocity, which can be in the region of 200ft per second. This heated gas passes from the fan, along the length of the autoclave, between the inner and outer walls of the vessel and is ‘bounced’ back by deflectors built into the inside of the door; over the part and back to the fan. Fan motors can be located either outside the vessel which necessitates a pressure seal on the fan driveshaft, or inside the pressure chamber, meaning the motor is subjected to temperature. Either design involves additional manufacturing cost and can be a maintenance nightmare.

With the exception of the pressure vessel and vacuum bag venting, the total concept is very similar to a hot bonding operation. As such, there is no need for a complex computer-based control system, as used on an autoclave.

The temperature control is provided by a repackaged hot bonder which is programmed in the same user-friendly way as the standard HCS9200B Dual Zone unit. This has the advantage that any technician who has used the hot bonder can be very easily trained to operate the RepairClave.

By utilising a dual, or two zone controller, it is possible to perform two independent repairs at the same time, or to ‘slave’ the two zones in order to complete a larger repair, using either two single zone blankets, or one-, two-zone heater.

The same heater blankets when not being used in the RepairClave can be utilised for a standard hot bonding procedure, which maximises their usefulness. A second panel incorporates the valves to adjust and control the pressure and vacuum levels, the latter of which can be vented to atmosphere as required.

One application which has been an instant success is the use of the RepairClave to repair helicopter blades. The repair procedure on certain types calls for pressure in excess of that which can be achieved using vacuum and the traditional method of using pressure ‘bladders’ is far from being perfect.

Both helicopter operators and manufacturers have been quick to see the advantages of such a system, with one unit having been installed and commissioned in South America, and a second for a major American producer. The RepairClave has since been approved for use in repair of the latest generation composite aircraft.

The sky’s the limit

Various sizes of the units have produced, including a 10ft x 36” diameter, 30ft x 36” diameter, 3ft x 48” diameter, which will be used in the research of composite materials to be used on a new regional jet project and a 30ft x 48” diameter.

As the RepairClave concept is a relatively simple one when compared to the complexity of an autoclave; it has significantly fewer components and therefore a much-reduced weight. This means that the unit can be shipped as a complete assembly which simplifies transportation to site and more importantly allows hydrostatic testing, to check the integrity of welding, and confirm compliance with the ASME code, before shipment.

Unlike with a permanent autoclave installation, there is rarely any need to provision for special civil foundations and in fact lockable ‘castors’ are usually fitted which makes repositioning within a facility a relatively simple and straightforward operation.

Last but not least, for reasons mentioned previously there is a huge cost saving over the traditional equipment and therefore those large, previously ‘out of capability’ repairs may in the future become common practice.

www.heatcon.com

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