Although not usually regarded as true sandwich cores, there are a number of thin, low-density 'fabric-like' materials which can be used to slightly lower the density of a single-skin laminate. Materials such as Coremat and Spheretex consist of a non-woven 'felt-like' fabric full of density-reducing hollow spheres. They are usually only 1-3mm in thickness and are used like another layer of reinforcement in the middle of a laminate, being designed to 'wet out' with the laminating resin during construction. However, the hollow spheres displace resin and so the resultant middle layer, although much heavier than a foam or honeycomb core, is lower in density than the equivalent thickness of glass fibre laminate. Being so thin they can also conform easily to 2-D curvature, and so are quick and easy to use.
Published courtesy of David Cripps, Gurit