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Toolside and Bagside breathing are long established techniques. They are used to remove air and volatiles from a composite laminate while curing under a vacuum bag. BAE Systems has successfully employed Dahltexx during their recent UAV technology demonstrator project in support of the UK ASTRAEA programme.
PHILIP LUNN, EUROPEAN TECHNICAL MANAGER, AIRTECH ADVANCED MATERIALS GROUP
The project, supported by AirtechAdvanced Material Group,developed the use of LiquidResin Infusion for manufacture of UAVstructures. The ASTRAEA programme,part funded by the NWDA, aims toopen the airspace to UnmannedAircraft Systems. The programmeincluded research into thequalification of affordable aircraftstructures and was based on theHERTI UAV wing design.
Construction of the wing skinsfeatures non-crimped bi-axiallyoriented carbon fabrics around apolymer foam core, infused with anepoxy resin system. A common infusion technique is to drill and groove the foam core to promote theflow of resin. In order to reduce resinweight and avoid restrictions onmechanical performance created bydrilled and grooved core, thistechnique was not used on the project.The lack of resin flow promotion inthe core material provided a challengeto ensure complete fibre wet-out overthe entire part.
To ensure complete fibre wet-out and avoid any surface porosity the BAE Systems Engineers employed Toolside and Bagside breathing techniques with the Airtech Dahltexx semi-permeable membrane. Dahltexx is a gas permeable fabric which due to its micro-porous structure will allow the passage of air and restrict the flow of resin. The liquid resin infusion was completed at room temperature with standard workshop vacuum connected through an Airtech RB451 vacuum reservoir. After infusion curing was completed at elevated temperature in an oven.
Trial moulding made without Toolside or Bagside breathing resulted in surface porosity over the part surface. Subsequent mouldings with Dahltexx used for Toolside and Bagside breathing resulted in zero porosity over the tool side of the part and zero porosity around the complex features of the access panel and aileron features on the bag side of the part. These Wing skins then passed NDT examination to Eurofighter Typhoon standard with a Grade A acceptance criteria.
The project delivered a reduction in labour of around 25%, reduction in raw materials of 30%, weight reduction of 30% and achieved 1.3 x ultimate load in static test. The success of the project has been to demonstrate affordable manufacturing of composite airframe structures with the LRI process employing Dahltexx for Tool side and bag side breathing, with opportunities for its use in present and future projects.