Both polyurethane isocyanate and polystyrene can be foamed with suitable gassing agents to form rigid shell structures. If the foaming is unrestricted then it becomes open celled, but if restricted within shell moulds considerable pressures are created and the foam becomes compacted, with a hard outer skin which will conform to every fine detail in the mould. Self-supporting chair shells, imitation mouldings and wood carvings are made by these methods. The polyurethane foam is the more expensive of the two but will accept staples if upholstered, whereas polystyrene must have tacking strips applied. An example of the open-celled polystyrene foam is the lightweight ceiling tile.
Recent developments in lightweight sandwich boards use an open-celled ABS foam with outer layers of compacted foam.
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