Non-autoclave processing of honeycomb sandwich structures: Skin through thickness air permeability during cure

In non-autoclave processing of sandwich structures, the pressure level inside the honeycomb becomes a critical process parameter, which depends on the permeability to air of the upper skin. In this work, prepreg and adhesive permeabilities were determined separately and in combination through a falling pressure method. We showed that a range of initial skin through thickness air permeability could be achieved by perforating the prepreg plies, the adhesive layer or a combination of both. A corresponding range of achievable pressure was measured inside the honeycomb. The change in the through thickness permeability with time during the cure cycle was determined for each of the studied cases. The adhesive layer was identified as the element that most reduces the initial through thickness air permeability in skin manufacturing.