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The figures below give the shear strength and compressive strength of some of the core materials described, plotted against their densities. All the figures have been obtained from manufacturers’ data sheets.
As might be expected, all the cores show an increase in properties with increasing density. However, other factors, besides density, also come into play when looking at the weight of a core in a sandwich structure. For example, low density foam materials, while contributing very little to the weight of a sandwich laminate, often have a very open surface cell structure which can mean that a large mass of resin is absorbed in their bondlines. The lower the density of the foam, the larger are the cells and the worse is the problem. Honeycombs, on the other hand, can be very good in this respect since a well-formulated adhesive will form a small bonding fillet only around the cell walls.
Finally, consideration needs to be given to the form a core is used in to ensure that it fits the component well. The weight savings that cores can offer can quickly be used up if cores fit badly, leaving large gaps that require filling with adhesive. Scrim-backed foam or balsa, where little squares of the core are supported on a lightweight scrim cloth, can be used to help cores conform better to a curved surface. Contour-cut foam, where slots are cut partway through the core from opposite sides achieves a similar effect. However, both these cores still tend to use quite large amounts of adhesive since the slots between each foam square need filling with resin to produce a good structure.
In weight-critical components the use of foam cores which are thermoformable should be considered. These include the linear PVC’s and the SAN foams which can all be heated to above their softening points and pre-curved to fit a mould shape. For honeycombs, over-expanded forms are the most widely used when fitting the core to a compound curve, since with different expansion patterns a wide range of conformability can be achieved.
Published courtesy of David Cripps, Gurit
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