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5.4.1.6 Other thermoplastics As new techniques develop for the blowing of foams from thermoplastics, the range of expanded materials of this type continues to increase. Typical is PEI foam, an expanded polyetherimide/polyether sulphone, which combines outstanding fire performance with high service temperature. Although it is expensive, this foam can be used in structural, thermal and fire protection applications in the service temperature range -194°C (-320°F) to +180°C (+355°F). It is highly suitable for aircraft and train interiors, as it can meet some of the most stringent fire resistant specifications. 5.4.2 Honeycombs Honeycomb cores are available in a variety of materials for sandwich structures. These range from paper and card for low strength and stiffness, low load applications (such as domestic internal doors) to high strength and stiffness, extremely lightweight components for aircraft structures. Honeycombs can be processed into both flat and curved composite structures, and can be made to conform to compound curves without excessive mechanical force or heating. Thermoplastic honeycombs are usually produced by extrusion, followed by slicing to thickness. Other honeycombs (such as those made of paper and aluminium) are made by a multi-stage process. In these cases large thin sheets of the material (usually 1.2x2.4m) are printed with alternating, parallel, thin stripes of adhesive and the sheets are then stacked in a heated press while the adhesive cures. In the case of aluminium honeycomb the stack of sheets is then sliced through its thickness. The slices (known as ‘block form’) are later gently stretched and expanded to form the sheet of continu- ous hexagonal cell shapes. In the case of paper honeycombs, the stack of bonded paper sheets is gently expanded to form a large block of honeycomb, several feet thick. Held in its expanded form, this fragile paper honeycomb block is then dipped in a tank of resin, drained and cured in an oven. Once this dipping resin has cured, the block has sufficient strength to be sliced into the final thicknesses required. In both cases, by varying the degree of pull in the expansion process, regular hexagon- shaped cells or over-expanded (elongated) cells can be produced, each with different mechanical and handling/drape properties. Due to this bonded method of construction, a honeycomb will have different mechanical properties in the 0° and 90° directions of the sheet. While skins are usually of FRP, they may be almost any sheet material with the appropri- ate properties, including wood, thermoplastics (eg melamine) and sheet metals, such as aluminium or steel. The cells of the honeycomb structure can also be filled with a rigid foam. This provides a greater bond area for the skins, increases the mechanical properties of the core by stabilising the cell walls and increases thermal and acoustic insulation properties. 42 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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