Manufacturing of composite tubes using a simulated autoclave technique

In this paper, an alternative method to autoclave curing of composite tubes having a square cross section with rounded corners is demonstrated. The method consists of using a high thermal expansion silicone rubber material external to the tube layup. The composite tube is laid up on an aluminum mandrel, and the composite layup and silicone are contained in a metallic mold. At the cure temperature of 350°F, the high expansion of the silicone creates an external pressure on the composite. The ability to control the magnitude of the pressure to any desired value was incorporated by designing a special outer mold that allows gradual adjusting of its volume. A companion analytical model has also been presented as an aid in designing the mold to achieve this pressure. Tubes manufactured using this technique at pressures of 0.69-1.03 MPa (100-150 psi) were found to be of good quality and had reasonably uniform wall thicknesses. In particular, this technique eliminates problems with excessive pressure and fiber bridging at the corners previously observed when using expansive mandrels internal to the layup.