Non-autoclave processing of sandwich structures: the role of prepreg through thickness air permeability

Non-autoclave processing of honeycomb sandwich structures generally leads to poor compaction and high porosity of the skins along with a decreased skin-core adhesion. Air permeability of the skins, a critical parameter of low pressure processing, is often increased by perforating the prepregs in a patterned way, before curing of the second skin. This procedure, frequently designated as "spiking", provides an air path for the honeycomb cells voiding. Prepreg through thickness air permeability was characterised at room temperature and its evolution monitored during cure. Several spiking configurations were tested and their role on skin-core adhesion and skin quality was evaluated. Prepreg air permeability controls skin-core bonding through the pressure drop in the honeycomb cells and potential outgassing of the adhesive layer. An optimal range of skin permeability was found to be between 5×10-14 cm2 and 3×10-12 cm2, which corresponds to a range of 200-600 mbar of air pressure in the honeycomb.