JEC COMPOSITES MAGAZINE - Issue #53 - November/December 2009 - 54

KNOW-HOW Localized heating Positive pressure repairs without an autoclave Autoclaves are used to manufacture most of the composite structures found in today’s aircraft. Repairing such structures in an autoclave is both time-consuming and expensive. To address this issue, Heatcon® Composite Systems developed a system that uses only localized heat and shop air to effect the repair. - the air circulation fan which is capable of producing air movement velocities up to 60 metres per second, - the heating elements which must be capable of bringing the massive internal volume of the autoclave up to cure temperature, and - the complex computer-controlled system. In addition to this, civil engineering considerations have to be taken into account, as the foundation not only has to be strong enough to support the weight of the actual autoclave, but it also needs to support the weight of the vessel when filled with water so that periodic hydro-testing can be carried out for pressure vessel certification purposes. It should be possible to simulate autoclave manufacturing conditions during the repair process in order to achieve a structurally-sound repair and return the damaged part to its “as manufactured” condition. The Hot Bonder has been used extensively for this purpose since the mid-1980s. A silicone rubber heater blanket was an easy solution to provide the necessary heat for curing, but compaction of the materials became a challenge as it was impossible to use positive pressure for performing in situ repairs. A vacuum (negative pressure) was the obvious alternative to positive pressure, seeing that it not only compacted the materials and removed the volatile gases from the repair area, but meant that repairs could be made in any orientation. The only drawback with this system is that even if an absolute vacuum is achieved, it cannot simulate more than approximately 1 bar of positive pressure. This “pressure” is sufficient to repair most common, everyday damage to current aircraft. However, there are instances, particularly for large repairs or repairs on critical parts, where more compaction is called for than can be created by using vacuum. This scenario is likely to happen more frequently with the introduction of new composite aircraft and the ever-expanding use of monolithic structures. In this case, the only alternatives up to now have been to return the part to the autoclave, or use some sort of external pressure in conjunction with vacuum. Returning the part to the autoclave is not always practical due to availability and expense factors. TOM LANE, MANAGING DIRECTOR (EUROPE), HEATCON® COMPOSITE SYSTEMS y taking a closer look at the current autoclave and Hot Bonder processes, including their drawbacks and advantages, Heatcon® Composite Systems was able to develop an innovative new solution – the RepairClave – to resolve the repair issue (Figure 1). Autoclaves are used to manufacture most composite structures used in newer generation aircraft. The autoclave provides control of the three main elements for curing a part, namely the temperature, time and pressure, which is essential for compacting the layers of material during the curing process. Fig. 1: RepairClave. Looking for an alternative to the autoclave The autoclave is a very complex piece of equipment which is usually designed for a manufacturing process and therefore tends to be both expensive to buy as capital equipment, and expensive to operate. The main factors influencing cost are: - the double-shell construction which is required for effective air circulation, jec composites magazine / No53 November - December 2009

JEC COMPOSITES MAGAZINE - Issue #53 - November/December 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #53 - November/December 2009