A composite product is formed from a honeycomb panel having an array of hexagonal tubular cells which clamped between two press plates where a layer of a foaming agent is applied to only the bottom press plate such that the foaming agent expands into the cells. An array of grooves is provided in the top press plate and connected at one or both edges of the plate to a source of vacuum so as to draw air through the grooves and cells. A sheet of a polyethylene material having the characteristics that it is permeable to air, substantially impermeable to liquid and resistant to bonding to the foam is applied onto the press plate to prevent the foam entering the grooves and bonding to the press plate while allowing the air to be drawn from the cells.