Components such as propeller or helicopter rotor blades, wings, or structural members consist of an outer shell of thin metal, plastic or plywood, with an internal supporting filling of low density (about 4 lb./cu. ft.) and having a closed cellular structure formed by expanding a thermoplastic material such as polyrinyl chloride, incorporating a hardening agent such as phenol formaldehyde or urea resin or a paint‐hardening material. The manufacturing process is carried out in two stages, the mix for the internal filling being first placed in a mould similar in shape to, but smaller than, the outer shell, and then heated sufficiently to cause the mix to fill the mould, from which it is then removed and allowed to cool to a solid‐like mass having a similar shape to the outer shell. The cooled mass is then inserted in the shell and heating continued until the gases liberated expand the mass to fill the shell and apply internal pressure thereto, to impart the desired rigidity to the member.
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