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and acts as an insulating and ablative layer in a fire, with the core charring slowly, allow- ing the non-exposed skin to remain structurally sound. It also offers positive flotation and is easily worked with simple tools and equipment. Balsa core is available as contoured end-grain sheets 3 to 50mm thick on a backing fabric, and rigid end-grain sheets up to 100mm thick.These sheets can be provided ready resin-coated for vacuum-bagging, prepreg or pressure-based manufacturing processes such as RTM. One of the disadvantages of balsa is its high minimum density, with 100kg/m3 being a typical minimum. This problem is exacerbated by the fact that balsa can absorb large quantities of resin during lamination, although pre-sealing the foam can reduce this. Its use is therefore normally restricted to projects where optimum weight saving is not required or in locally highly stressed areas. 5.4.2.6 Other Core Materials 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 CorematTM and SpheretexTM 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. 5.4.3 Design considerations 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 part-way 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. GTC-6-0417 - 45PDF Image | GUIDE TO COMPOSITES
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