Old is new in vacuum bonding honeycomb sandwich

In Care and Repair of Advanced Composites (published by the Warrendale, Pa.-based Society of Automotive Engineers in 1997), Dr. Keith Armstrong, with a Ph.D in Adhesion Science and 24 years of experience in composite fabrication and repair at British Airways (Waterside, Harmondsworth, U.K.), writes about the problem of “achieving adequate bonding pressure for film adhesives with vacuum alone when 14.7 psi (1013.5 mbar) is already in the honeycomb.” He continues, “.... as the vacuum pump begins to extract air from the bag, some air is removed from the honeycomb cells, especially near the edge of the panel. However, as the vacuum pressure increases, the film adhesive forms a seal over the ends of the honeycomb cells, and air is no longer extracted.”

In British Airways testing, a 12-inch/0.3m-square test panel would not bond in the center area due to internal pressure when heated under vacuum-only conditions. Armstrong says the pressure inside the honeycomb cells will increase by a factor of 1.5 during a 120°C/250°F cure, and even higher for a 180°C/350°F cure, and advises reducing the pressure inside the honeycomb core accordingly. For a 120°C cure, where 14.7 psi/1 bar of vacuum is applied to the honeycomb sandwich, the pressure inside the core would need to be reduced to 5.8 psi/400 mbar) before heat is applied. Armstrong explains, “The technique for doing this with nonperforated honeycomb is to use a suitable fabric on one side of the sandwich between the adhesive and the honeycomb. British Airways selected a nonwoven polyester monofilament fabric, sometimes called a positioning cloth, which was commonly used as a carrier in film adhesives and featured low moisture uptake. He stresses that whatever fabric is chosen, it should not be thicker than 0.003 inch/0.076 mm (two layers of the British Airways fabric totaled 0.0015 inch/0.038 mm) and that the film adhesive should consist of one 0.085 lb/ft³ (1.36 kg/m³) layer or two 0.06 lb/ft³ (.096 kg/m³) layers to ensure an adequate amount of adhesive to absorb the fabric and to form a sufficient adhesive fillet between the skin and the honeycomb cell ends.