Archive for August, 2009

There are about as many members in this family as there are in the foam family of materials, and just as you might expect, the differences between them are rather wide. Most honeycombs are a nested array of hexagonal cells, made from any one of the many standard papers, woven fabrics, plastic or metal foils. Many non-hexagonal honeycombs have been used commercially, but the hexagonal cores exhibit both higher strength and a better balance of properties than most other configurations.

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This is a family of materials which includes both rigid and flexible materials, a great many choices of density and several different chemical versions. In the composites realm, we only use the rigid foams. Clark foam was used widely for manufacturing surf boards, but is not offered as a precision sliced material for use in sandwich core structures.

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This is the material used in aircraft wings and control surfaces, as well as surfboards. It was chosen because its strength properties are well matched to the need of these particular structures, and because it can be hot-wire contoured without generating poisonous gases. It has the strong disadvantage of being soluble in Styrene monomer, which is a major part of polyester and vinyl ester resins, and it is softened badly by exposure to petrol and several other solvents. It is processed by expansion or extrusion.

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The word “syntactic” simply means that the foam is made by mixing micro balloons, ceramic spheres, or other lightweight aggregate, with a resin system. This results in the density of the material before cure being the same as the density after cure. This is different from most other foams, which are expanded during cure to make the final density much lower than the density of the mixed resin system just prior to the expansion, or “blowing”, which takes place as cure proceeds.

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These materials are based on the same chemical family as the familiar garbage bags, plastic pipe and plastic films in common use, but are produced quite differently. The, manufacturing process is more difficult than those used to produce most other foams, and the materials are consequently higher priced. Several are available, all made overseas and imported as finished core materials.

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A number of foamed plastic materials are widely used in boat hulls, as well as in home- built aircraft. They are a bit higher in cost, but are lighter and easy to work with. Each of these material families has a set of mechanical properties, physical properties, working and handling characteristics, and a cost structure all of their own. In addition to the differences from one chemical type to another, there may also be major differences between the low-density and high-density members of the same family. Cost and performance will need to be considered for each case.

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Some of the most commonly used and lowest cost cores are any of the several light woods. The most popular is Balsa, but substantial use has also been made of Spruce, Western Red Cedar and many others.

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The structural sandwich is generic name for a group of materials in which we build the composite using a thick, low density core material, faced on both surfaces with a thin, high-strength skin, or facing. The sandwich has to have a very strong and rigid bond which holds the facings in good contact with the core. The skins must also be as flat as possible.
The strength and rigidity of this attachment is so important that a structural sandwich made with rubber cement or other soft adhesives does not qualify as a sandwich, with predictable properties.

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